1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petro- chemical industry or gasoline components with high resistance to engine knock. Demand for aromatics is growing more rapidly than the supply of feedstocks for aromatics production. Moreover, increased gasoline upgrading necessitated by environmental restrictions and the rising demands of high-performance internal-combustion engines are increasing the required knock resistance of the gasoline component as measured by gasoline "octane" number. A catalytic reforming unit within a given refinery, therefore, often must be upgraded in capability in order to meet these increasing aromatics and gasoline-octane needs. Such upgrading could involve multiple reaction zones and catalysts and, when applied in an existing unit, would make efficient use of existing reforming and catalyst-regeneration equipment.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffinde-hydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke. Continuous catalytic reforming, which can operate at relatively low pressures with high-activity catalyst by continuously regenerating catalyst, is effective for dehydrocyclization.
U.S. Pat. No. 3,287,253 (McHenry, Jr. et al.) discloses a reforming process comprising three different reforming zones. The first zone contains a non-acidic, nonhalogen-retaining catalyst, the intermediate stage contains a catalyst comprising an acidic support which promotes isomerization and the final stage is directed to dehydrocyclization of paraffins. The sequence of stages of McHenry, Jr. et al. thus contrasts sharply with that of the present invention.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed. Nevertheless, this dehydrocyclization technology has been slow to be commercialized during the intense and lengthy development period. The present invention represents a novel approach to the complementary use of L-zeolite technology.
U.S. Pat. No. 4,645,586 (Buss) teaches contacting a feed with a bifunctional reforming catalyst comprising a metallic oxide support and a Group VIII metal followed by a zeolitic reforming catalyst comprising a large-pore zeolite which preferably is zeolite L. The deficiencies of the prior art are overcome by using the first conventional reforming catalyst to provide a product stream to the second, non-acidic, high-selectivity catalyst. There is no suggestion in Buss of the three-zone reforming process of the present invention.
U.S. Pat. No. 4,985,132 (Moser et al.) teaches a multizone catalytic reforming process, with the catalyst of the initial zone containing platinum-germanium on a refractory inorganic oxide and the terminal reforming zone being a moving-bed system with associated continuous catalyst regeneration. However, there is no disclosure of an L-zeolite component.
U.S. Pat. No. 5,190,638 (Swan et al.) teaches reforming in a moving-bed continuous-catalyst-regeneration mode to produce a partially reformed stream to a second reforming zone preferably using a catalyst having acid functionality at 100-500 psig, but does not disclose the use of a nonacidic zeolitic catalyst.
A reforming process comprising three stages with a bifunctional catalyst followed by a zeolitic catalyst followed by a bifunctional catalyst is taught U.S. Pat. No. 5,885,439 (Glover), but the physical mixture used in the present process is not disclosed.